1. Field of the Invention
The present invention relates to an electrophotographic process and apparatus therefor, and more particularly to an electrophotographic process for achieving repeated image formation by means of a photosensitive member essentially composed of a conductive layer, a photoconductive layer and an insulating layer and an apparatus adapted for executing such process.
2. Description of the Prior Art
For the purpose of obtaining an image of a high contrast there is already known and commercially employed an electrophotographic process using a photosensitive member essentially consisting of a conductive layer, a photoconductive layer and an insulating layer and comprising, for example, a primary charging step for subjecting the surface of said photosensitive member to a uniform charging of a predetermined polarity, an image exposure step for exposing said surface to a light image, a secondary charging s applying, approximately simultaneously with said image exposure, a corona discharge containing a component of a polarity opposite to that of the primary charging, namely a DC corona discharge of said opposite polarity, an asymmetric AC corona discharge or an corona discharge, and a whole surface exposure step for uniformly illuminating the surface of said photosensitive member to form an electrostatic latent image, followed by the development and transfer of the thus formed latent image, thereby obtaining a reproduced image of said light image.
Also, even if such high-contrast image cannot be obtained, the electrophotographic processes utilizing a photo-sensitive member essentially composed of a conductive layer, a photoconductive layer and an insulating layer are known to be suitable for repeated use of the photosensitive member, as the surface thereof can be rendered physically and chemically durable.
Such photosensitive member has, however, been found to result in a significant loss in image contrast in the repeated image formation in case of a certain combination of the polarity of the developer, of the secondary charging and of the corona discharge for image transfer.
Such contrast loss has been a serious problem in certain processes, as such combination is, for example, indispensable for achieving a reversal development or for removing the excess liquid in a liquid development.
The above-mentioned drawback will be more specifically explained in an example of an electrophotographic process involving a reversal development shown in the attached drawings.
In such process schematically shown in FIG. 1, a photosensitive member A essentially composed of a conductive layer a1, a photoconductive layer a2 and an insulating layer a3 is at first subjected to a primary charging, for example, with a positive corona discharge in case said photoconductive layer a2 is of an N-type such as a CdS-binder system (FIG. 1(a)), then subjected to the exposure of a light image (L and D respectively indicating light and dark areas) simultaneously with a secondary charging with an AC corona or a DC corona containing a component of a polarity opposite to that of said primary charging (FIG. 1(b)), and is subjected to a whole surface exposure to obtain an electrostatic latent image wherein the light area L and the dark area D are respectively charged negatively and positively (FIG. 1(c)). The latent image thus obtained is rendered visible by reversal development with a dry or liquid developer T containing positively charged toner particles (FIG. 1(d)), and the thus developed image is transferred onto a transfer sheet or paper P by superimposing said transfer sheet P on the surface of the photosensitive member A and applying a negative corona discharge from the back side of said transfer sheet P (FIG. 1(e)). Successively, the photosensitive member utilized for the image formation is prepared for the succeeding imaging cycle by a cleaning step for eliminating the developer remaining on the surface of said photosensitive member.
In such process the contrast of the electrostatic latent image gradually decreases in the repeated use of the photosensitive member, and this phenomenon is attributed by the present inventors to the following facts.
In the image transfer step of the above-explained process the negative charging, through the transfer sheet, of the surface of the photosensitive member containing an N-type photoconductive layer induces a positive charge at the interface between the conductive layer a1 and the photoconductive layer a2, thus creating an electric field across the photoconductive layer.
The electrons which are injected from the conductive layer and retained at the interface between the photoconductive layer and the insulating layer in case of normal positive charging of the surface of the photosensitive member are repelled from the photoconductive layer to the conductive layer by the above-mentioned electric field, thus leaving positive spatial charges or positive holes in the photoconductive layer.
Said positive holes become retained in the traps in a gradually increasing number with the lapse of time.
Particularly in the case of a photoconductive layer containing an elevated number of barriers, such as a layer composed of ZnO or CdS and a binder, the above-mentioned positive holes are captured in deep traps and do not easily recombine.
Such trapped positive holes trap, in the vicinity thereof, the electrons injected at the succeeding primary charging, and likewise hinders the movement of the injected electrons toward the interface between the photoconductive layer and the insulating layer.
For this reason the electron injection to the interface between the photoconductive layer and the insulating layer becomes deficient, resulting in a lowered contrast in the succeeding imaging cycles.
Furthermore, such trapped positive holes, being dependent on the image exposure in the preceding image forming cycle, appear as a memorized image in the succeeding imaging cycle.